The current invention is directed to a device which reduces the electromagnetic interference produced by an optical transceiver used in optical data links between data processing equipment.
Optical transceivers are commonly used in communications equipment. A transceiver comprises an optical transmitter and an optical receiver, both housed in a common enclosure. The transmitter converts an electrical input into a temporally modulated optical signal coupled to an optical fiber. The receiver accepts an optical signal having a modulation function, and converts it to an electrical voltage. Some versions of this receiver also recover a clock to in addition to the recovered data. The transmitter and receiver generally operate at the same data rate. In the prior art, typical data rates are 155 Mbps (million bits per second) for oc-3 rates, 622 Mbps for oc-12 rates, and 2400 MBps for oc-48 rates. U.S. Pat. No. 4,979,787 by Lictenberger discloses an optical interface for receiving from and transmitting to an optical fiber. U.S. Pat. No. 5,113,466 by Acarlar et al, U.S. Pat. No. 5,011,246 by Corradetti et al, disclose optical transceivers, but do not speak to the problem of preventing EMI emissions. U.S. Pat. No. 5,337,396 describes a conductive plastic housing for a transmitter or a receiver which provides electrical contact to the printed circuit board ground pins, but does not address the problem of making a shielding contact to an enclosure, or reducing EMI coupling to the optical cables.
A first object of the invention is to reduce the EMI (Electro-Magnetic Interference) emissions of a transmitter by providing a grounded reference for unbalanced transmitter currents. A second object of the invention is to reduce the EMI emissions of a receiver by minimizing unbalanced clock currents flowing in the recovered clock output pins. A third object of the invention is to reduce the EMI emissions of a receiver by detuning the structure housing the receiver. A fourth object of the invention is to provide a chassis reference to an internal receive ferrule and an internal transmit ferrule through a first extension shield which is connected electrically to a collar shield, which is in contact with the chassis reference.
A prior art oc-12 transceiver conducts differential and common mode currents at a fundamental rate of 622 Mhz, and a prior art oc-48 transceiver conducts these currents at a fundamental rate of 2400 Mhz. A prior art transceiver has a physical length roughly equal to that of a quarter wave antenna for approximately 3 Ghz electromagnetic waves. A fundamental rate 622 Mhz square wave signal from an oc-12 data link has a 5th harmonic in this same 3 Ghz range. Hence, the 5th harmonic of a 622 Mhz transmit or receive electrical signal will excite a quarter wave antenna response in the 3 Ghz range from the prior art transceiver, affording gain and a nearby aperture for radiation of this signal. The present invention provides for the reduction of EMI by using pre-existing internal optical elements as shield elements, and by detuning the quarter wave antenna property of the transceiver.